Mstislav Davydovich is the son Davyd Rostislavich, Prince of Smolensk. According to V.N. Tatishchev, he is the same person with Boris Davydovich of Polotsk, in this case also the prince of Polotsk between 1180 (the last mention of Vseslav Vasilkovich of Polotsk ) and 1184 (the first mention of Vladimir of Polotsk) / 1186 (mention of Vseslav Dutsky, sometimes identified with Vseslav Vasilkovich Polotsky).
In 1184, or earlier, he married the daughter of Polovtsian Khan Tolgiy, there is no information about the children. 
- ^ a b Домбровский Д. Генеалогия Мстиславичей. Первые поколения (до начала XIV в.) / Пер. с польского и вступ. слово к рус. изд. К. Ю. Ерусалимского и О. А. Остапчук. — СПб.: ДМИТРИЙ БУЛАНИН, 2015. — 880 с. — С. 501—506.
- ^ In the chronicle, Tolga's son-in-law is named only by his patronymic, it is generally accepted that he was Mstislav Davydovich was such. In the opinion of D. Dombrovsky the daughter of Tolgiy, married Izyaslav DavydovichИзяслав Давыдович.